Dusting device

ABSTRACT

A device for dusting flat products such as printed matter with a powder includes a powder box open toward a product transport surface on which a plurality of the products which are to be dusted are moved. The powder box has a powder nozzle arrangement including a first plurality of compressed air/powder delivering nozzles arranged transversely to a product transport direction at an upstream edge of the powder box, and a second similar nozzle arrangement at a downstream edge of the powder box, the two nozzle arrangements cooperating to product an air current directed towards the product transport surface. First and second suction nozzle arrangements are also similarly provided, respectively downstream of the first air nozzle arrangement and the second air nozzle arrangement, both suction nozzle arrangements also being oriented transversely to the product transport surface. A first portion of the back wall of the powder box, assigned to a downstream portion thereof, is disposed to be at a greater distance from the product transport surface than is a second portion of the back wall assigned to a relatively upstream box portion adjacent to the first and second air nozzle arrangements.

FIELD OF THE INVENTION

The invention relates to a device for dusting flat products, especiallyprinted products, with a powder.

A dusting device of known kind is described in DE-PS 22 07 983. In thecase of that device, an upstream and downstream air curtain ensures thatno powder dust leaves the actual treatment chamber. Removal of the airin the interior by suction ensures that the air curtains divide cleanlyand without turbulence at their free end adjacent to the producttransport plane and that excess powder is continuously drawn off fromthe treatment chamber. Such dusting devices have proved successful inpractice.

In modern printing machines, distinctly increased transport speeds areused and, in order to ensure that the printed products are neverthelessdusted sufficiently well that they also continue not to stick together,it is per se necessary to have a powder box that has been enlarged inthe direction of transport.

SUMMARY OF THE INVENTION

It has been realized that a powder box having only small dimensions willsuffice, despite the increased transport speeds, if, on the one hand,the powder density in the powder/air mixture is increased whilst at thesame time, however, by improving the dynamic sealing of the powder boxand improving the flow relationships in the powder box itself, unduesoiling of the surroundings, as would otherwise result from increasingthe powder density, is counteracted.

This becomes possible according to the invention by means of a dustingdevice having the features specified in claim 1.

A powder box comprising two box portions having cover walls at differentdistances from the product transport plane can be obtained with littleexpenditure.

Advantageous developments of the invention are indicated in thesubclaims.

As a result of the development of the invention according to claim 2,despite the increased powder density in the interior of the powder box,no undesirable powder deposit layers are obtained on the ceiling of thedownstream box portion. Such layers could fall off in an uncontrolledmanner and render high-quality printed products unusable.

When the products to be dusted are transported at very high speeds, theytransmit impulses to the powder/air mixture in the powder box. By meansof the development of the invention according to claim 3 it is possibleto compensate for that entraining effect. Even better sealing of thedownstream end of the powder box is achieved in that manner.

According to DE-PS 22 07 983, described earlier, the downstream suctionnozzle arrangement is situated in front of the downstream air nozzlearrangement, viewed in the direction of transport. That side-by-sidearrangement results in very sharp deflection of the air in a small area.The development according to claim 5 provides for the downstream suctionnozzle arrangement to be situated in the end wall of the box above thedownstream air nozzle arrangement, thereby producing a less sharply bentcirculation which covers a larger area of the end of the box, that is tosay more a roller of air rotating counter to the transport directionthan a curtain of air meeting the transport surface at right angles.That applies particularly when the downstream air nozzle arrangement isinclined.

Other developments of the invention according to claims 6 to 8 serve toimprove the lateral sealing of the dusting device.

If a printed product flutters or undulates as it is moved through thedusting device, that has an adverse effect on the flow relationships inthe interior of the dusting device and on the dynamic sealing of thepowder box. The development of the invention according to claim 9ensures that the products to be dusted are aligned very precisely withtheir intended transport surface. The printed products, which areconsequently mechanically smooth as far as the powder/air mixture isconcerned, can therefore take, at most, only very small amounts ofunbound powder out of the powder box in addition to the powder adheringto the still wet printing inks.

The development of the invention according to claim 10 is againadvantageous with a view to improving the lateral sealing of the powderbox.

By means of the development of the invention according to claim 11 anadditional, electrostatic seal is obtained at the downstream end of thepowder box. The high-voltage source used to put the downstream airnozzle arrangement at high voltage is polarised in such a manner thatthe electric field generated drives powder particles back into theinterior of the powder box.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in detail below with the aid of a preferredembodiment and with reference to the drawings, in which:

FIG. 1 shows a vertical section through a dusting device for printedproducts, according to a preferred embodiment, which is arrangeddownstream of a drier at the upper end of an ascending transport pathsection in a printing machine;

FIG. 2 shows a transverse section through the dusting device shown inFIG. 1, along the line of section II--II therein; and

FIG. 3 shows an enlarged section through a part of a cover wall of thedusting device shown in FIGS. 1 and 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As best seen in FIG. 1, 10, 12 denote two chain guides in which conveyorchains 14, indicated merely schematically, are guided. The conveyorchains 14 carry aligned pairs of spaced lugs 16 which in turn carrydriving members, not shown, for printed products. The transport surfacealong which the printed products are moved is indicated by a dot-dashline at 18.

In FIG. 1, the upper end of an ascending transport path section and asection bending into the horizontal are shown. Arranged in the ascendingtransport path section is a drier 20, indicated merely schematically,which dries the wet printing inks by hot air and/or radiation.

In order to prevent the printed products from sticking together by meansof the printing inks, which are still tacky even downstream of the drier20, a dusting device, designated 22 overall, is provided downstream ofthe drier 20.

The dusting device has a powder box, designated 24 overall, comprisingan upstream box portion 26 and a downstream box portion 28.

The upstream box portion 26 is constructed to have in its upstream end acompressed air channel 30 of square cross-section which is equipped inits wall that faces the transport surface 18 with a plurality of nozzleswhich together produce an upstream air curtain 32 directed at rightangles towards the transport surface 18.

Provided downstream of and parallel to the compressed air channel 30 isa suction channel 33 having suction apertures pointing towards thetransport surface 18. Downstream of the compressed air channel 30 andupstream of the suction channel 33 viewed in the direction of transport,a powder channel 34 is provided to which a powder/air mixture issupplied from a diffuser 36 indicated merely schematically. The diffuser36 may be constructed as described, for example, in DE-OS 38 19 203, towhich reference is made in that respect.

The upstream box portion 26 is closed towards the transport surface 18by a wall 38 the distance of which from the transport surface 18 ismarked by "h".

The box portion 28 is altogether inclined with respect to the boxportion 26 by a small angle w. That inclination is selected with a viewto adaptation to the curved transport surface 18.

Adjoining the downstream end of the wall 38 is a wall 40 which extendsaway from the transport surface 18 at an angle of approximately 45°. Thedownstream end of the wall 40 is connected to a wall 42 which extendsparallel to the outer surface of the downstream box portion 28. The wall42 has a distance from the transport surface 18 that is distinctlygreater than that of the wall 38. The average distance corresponds tothe length "H".

The walls 40 and 42 are formed of an air-permeable microporous material,for example a sintered material produced from fine polyethyleneparticles.

The walls 40 and 42 are fastened to strips 44 which in turn are carriedby external boundary walls 46 of the downstream box portion 28. The flatspace situated between the external boundary walls 46 and the walls 40,42 is connected via a line 48 to a source of compressed air.

Box profiles 50 form the side walls of the downstream box portion 28.They are in communication with the interior of the box portion 28 via aperforated wall 52 and are connected via a line 54 to a vacuum source.The same applies to a box profile 56 of substantially triangularcross-section which forms the end of the box portion 28 and which is incommunication with the interior of the box via a perforated wall 58(see, for example FIG. 2) and is likewise connected to the line 54.

A compressed air channel 60 of rectangular cross-section is connected tothe sloping lower end of the box profile 56 in such a manner that itscross-sectional axis is at an angle of approximately 45° to thetransport surface 18. In the lower end wall facing the transport surface18, the compressed air channel 60 carries a plurality of nozzles, notshown, which together produce an air curtain 62 which meets the printedproducts at an angle of 45°, counter to the product transport direction,as they leave the dusting device 22. That air curtain is for the mostpart sucked away through the openings in perforated wall 58, so that theair curtain forms altogether a roller of air of relatively largediameter which rotates counter to the product transport direction.

As will be seen from FIG. 2, compressed air channels 64, 66 arepartitioned off by partition walls in the box profiles 50, whichcompressed air channels are again provided, in their walls lying at thebottom in FIG. 2, with a plurality of nozzles which, fed by a compressedair line 67, produce lateral air curtains 68, 70 directed towards thetransport surface 18.

In order to ensure smooth transport of the printed products withoutbuckling in the region of the dusting device 22, a sheet guide box,designated 72 overall, is provided therein beneath the transport surface18. The sheet guide box is closed towards the transport surface by aperforated plate 74 having fine openings, and is connected to a vacuumline 76.

As will be seen from FIG. 2, there may be provided at the box profiles50 sealing strips 78 which hang downward and extend into the immediatevicinity of the movement surface of the ends of the driving members, notshown, moved by the conveyor chains 14. Correspondingly, the sheet guidebox 72 may carry sealing strips 80 extending as far as the movementsurface of the fastening ends of the driving members.

The compressed air channel 60 is mounted on the powder box 24 in anelectrically insulated manner and is connected to the one terminal of ahigh-voltage source 82 which delivers a voltage of 7.5 kV. The otherterminal of the high-voltage source is connected to the powder box 24,thus generating an electric field from the compressed air channel 60 tothe powder box 24, which additionally draws powder particles into theinterior of powder box.

As will be seen from FIG. 3, the wall 40 and the wall 42 are fastened tothe strips 44 by screws 84. The strips 44 have passages 86 so that therear side of the microporous wall material is acted upon by compressedair throughout.

As shown in an enlarged cut-out 88, the wall material comprises verysmall particles 90 sintered together, which may consist, for example, ofpolyethylene. Such a material is mechanically self-supporting and has avery small pore size.

Although the present invention has been described and illustrated indetail, it should be clearly understood that the same is by way ofillustration and example only and is not to be taken by way oflimitation, the spirit and scope of the present invention being limitedonly by the terms of the appended claims.

I claim:
 1. A device for dusting flat products, especially printedproducts, with powder, comprising:a powder box which is open towards aproduct transport surface along which the flat products move in aproduct transport direction, said powder box having a back wall and sidewalls; a powder nozzle arrangement which is arranged in the powder boxtransversely to the product transport direction and which delivers apowder/air mixture towards the product transport surface; a first airnozzle arrangement arranged transversely to the product transportdirection at an upstream edge of the powder box, and a second air nozzlearrangement arranged transversely to the product transport direction ata downstream edge of the powder box, said first and second air nozzlearrangements cooperating to produce an air curtain directed towards theproduct transport surface; and a first suction nozzle arrangementprovided downstream of the first air nozzle arrangement and a secondsuction nozzle arrangement provided adjacent the second air nozzlearrangement, both the first and second suction nozzle arrangements beingoriented transversely to the product transport surface, wherein a firstportion of the back wall assigned to a downstream box portion is at agreater distance (H) from the product transport surface than is a secondportion of the back wall assigned to an upstream box portion which isadjacent to the powder nozzle arrangement.
 2. Dusting device accordingto claim 1, wherein:a back wall of the downstream box portion is formedof an open-pored micropore material having a surface which faces awayfrom an interior of the powder box and is acted upon by compressed air.3. The device according to claim 1, wherein:the second air nozzlearrangement produces an air curtain which has velocity componentsdirected counter to the product transport direction.
 4. The deviceaccording to claim 3, wherein:the air curtain produced by the second airnozzle arrangement is inclined at approximately 45° to the producttransport surface.
 5. The device according to claim 1, furthercomprising:a vacuum source; and wherein a downstream end wall of thepowder box is provided with openings located over the second air nozzlearrangement, which openings are in communication with the vacuum source.6. The device according to claim 1, further comprising:a vacuum source;and wherein the side walls of the downstream box portion are providedwith openings and are in communication with the vacuum source.
 7. Thedevice according to claim 1, further comprising:a source of compressedair; and wherein the side walls of the powder box have additional airnozzle arrangements that are in communication with a source ofcompressed air.
 8. The device according to claim 1, wherein:the sidewalls of the powder box comprise seals.
 9. The device according to claim1, further comprising:a sheet guide box which lies opposite the powderbox in relation to the product transport surface, the sheet guide boxhaving a guide wall which faces the product transport surface and isprovided with a plurality of apertures; and a vacuum source, incommunication with said plurality of apertures in the guide wall. 10.The device according to claim 9, wherein:the sheet guide box has sidewalls which are provided with seals pointing towards the producttransport surface.
 11. The device according to claim 1, wherein:thesecond air nozzle arrangement is electrically insulated from thedownstream box portion, and comprises a high-voltage source, whichprovides a voltage in the range 3-8 kB and is connected to thedownstream box portion and to the second air nozzle arrangement.